“The room feels stale, CO2 rises and the building has changed use. How do I work out whether the mechanical ventilation is actually adequate?”
Existing-building ventilation problems are rarely solved by reading one airflow value from a table. Occupancy, enclosure use, air distribution, system control, exhaust, make-up air and measurement quality all influence the engineering answer.
The practical answer
The Standard covers mechanical air-handling systems that ventilate buildings and car parks and ventilation intended to control odours, particulates and specified gases. It deliberately separates ventilation from thermal comfort, maintenance, natural ventilation and fire/smoke safety.
The 2024 document includes occupancy-based outdoor-air requirements, exhaust and replenishment air, discharge locations, vehicle facilities, contaminant monitoring and particular health-care functions. The relevant pathway depends on what the enclosure is used for.
For an existing building, the engineering task is to compare the current use and occupancy with the available system capacity, control sequence, distribution and commissioning evidence. Historical drawings alone may not describe what the system now delivers.
Where this Standard fits
- Outdoor-air requirements for occupied rooms
- Toilet, kitchen, plant-room and process exhaust
- Make-up and transfer air associated with exhaust systems
- Car parks, enclosed driveways, loading docks and repair shops
- CO and other contaminant monitoring used to control ventilation
- Existing buildings where occupancy or room use has changed
- A thermal-comfort assessment
- A maintenance Standard for air-handling equipment
- A natural-ventilation design method
- A fire and smoke-control Standard
- A declaration that any high CO2 reading means non-compliance
- A reason to ignore air distribution and only calculate total airflow
How should a mechanical ventilation question be worked through?
Start with the enclosure and contaminant source, then trace the complete air path. Do not begin with a fan size or a CO2 threshold.
A good result needs the equipment definition, loads, design method, fabrication, inspection and operating limits to describe the same real system.
Key engineering concepts to understand
Occupancy drives the question
The same room can require a different ventilation basis after conversion from office use to training, meeting, hospitality or higher-density use.
Outdoor air must reach the occupied zone
A central AHU outdoor-air quantity is not enough if distribution, terminal control or transfer paths prevent the intended ventilation reaching the space.
Exhaust needs replenishment air
A strong exhaust fan can perform poorly or create unwanted pressure effects if the source and amount of replacement air are not considered.
Control strategy matters
Systems that do not run continuously should operate to suit occupancy. VAV and demand-control sequences can make a theoretically adequate system underperform in practice.
Discharge and intake locations interact
Exhaust should not simply be discharged at the easiest location if recirculation or cross-contamination can occur.
Car-park ventilation is a dedicated pathway
Vehicle facilities involve distribution, control, contaminant monitoring, make-up air and discharge questions beyond a generic air-change calculation.
Detailed engineering case studies
The examples below show how the Standard changes a real engineering decision. They are not clause summaries or universal answers; each case starts with the equipment, task and evidence available.
Boardroom converted into a 30-person training room
A 70 m² boardroom becomes a training room. The existing VAV terminal was selected for occasional meetings, but the new room is occupied at high density for four-hour sessions.
AS 1668.2 provides the mechanical ventilation framework, but the assessment has to start with the new room use and actual ventilation path rather than the old drawing label.
What made the job difficult
- The room is fully occupied while cooling demand is sometimes low.
- The terminal box turndown may reduce primary air during mild weather.
- The central AHU outdoor-air setting is shared across the floor.
- Portable CO2 readings are high but were not logged with occupancy and outdoor concentration.
How the engineering review should proceed
- Confirm the current use and occupancy schedule and identify the relevant outdoor-air basis.
- Review the VAV minimum position, primary airflow and how central outdoor air is apportioned to the room.
- Measure terminal airflow and trend occupancy-related CO2 as supporting evidence under representative conditions.
- Check whether the remedy is terminal rebalancing, control change, central capacity increase or a combination.
The engineering output should state whether the existing system can support the new use, what changes are required and how the final performance will be commissioned. The room should not be approved or rejected from a single spot CO2 reading.
Commercial kitchen exhaust upgraded without checking replacement air
A tenancy installs a larger kitchen exhaust hood and fan to address smoke and odour complaints. After the upgrade, entry doors become difficult to open and cooking odours spread into the dining area.
The exhaust quantity cannot be reviewed in isolation. AS 1668.2 treats exhaust, make-up air, pressure and discharge as connected parts of the ventilation system.
What made the job difficult
- The new exhaust fan was selected from hood duty but no coordinated make-up-air review was completed.
- The tenancy is connected to a shared mall air system.
- Door opening pressure and infiltration vary with weather and mall operation.
- The discharge location may also contribute to odour recirculation.
How the engineering review should proceed
- Establish the total exhaust duty and identify the intended sources of replenishment air.
- Review pressure relationships between kitchen, dining area, tenancy and adjoining spaces.
- Check whether electrical or control interlocking is needed between exhaust and supply systems.
- Inspect discharge and nearby intake locations before assuming the only issue is fan capacity.
A durable solution may require coordinated supply/make-up air, balancing and control changes rather than another exhaust fan increase. The final system should be commissioned as an airflow and pressure system.
Basement car park with intermittent CO alarms and fan complaints
A residential basement car park has variable-speed exhaust fans controlled by CO sensors. Occupants report noise, the fans seem to run at odd times and one area has recurring high readings.
The question is not solved by checking total fan capacity. AS 1668.2 includes ventilation distribution, controls, contaminant monitoring, sampling points, make-up air and system operation.
What made the job difficult
- Sensor calibration history is incomplete.
- Several car-park areas are separated by walls and ramps.
- Make-up air paths have changed after security screens were installed.
- The BMS sequence has been modified more than once.
How the engineering review should proceed
- Map the car-park zones, fan system, make-up-air paths and sensor locations.
- Review sensor operation and calibration before treating every alarm as an airflow-capacity problem.
- Check distribution to stagnant areas and the impact of later building changes on air paths.
- Trend contaminants, fan speed and operating conditions to test the actual control sequence.
The assessment should separate sensor problems, control problems and genuine ventilation-distribution problems. A fan replacement may not be necessary if the root cause is blocked make-up air or poor monitoring coverage.
Common mistakes
The current use and occupancy should drive the assessment, not a superseded drawing label.
CO2 can be useful evidence, but it is not a substitute for defining the ventilation basis and measuring the air system.
Poor distribution can leave zones under-ventilated even when the fan total looks adequate.
Exhaust performance and pressure relationships depend on where replacement air comes from.
A system with adequate design capacity can still fail if it does not operate when the building is occupied.
Damper, VAV and demand-control changes should be verified with representative operating data.
Information to gather before making the decision
Current room uses and occupancy schedules
Mechanical drawings and airflow schedules
Outdoor-air intake arrangement
Supply, return, exhaust and transfer-air paths
Fan and AHU data
VAV or terminal minimum settings
BMS operating sequences and trends
Commissioning and balancing records
Measured airflows and pressures
CO2 or contaminant logs with occupancy context
Nearby exhaust discharge and air intake locations
Recent fit-outs or changes that alter air paths
What should happen next?
Related AS Applied content
Source basis & limitations
- The page intentionally does not reproduce Appendix A airflow tables, car-park formulae or detailed discharge provisions.
- AS 1668.1, AS 1668.4 and AS/NZS 3666.1 are treated as explicit interfaces because AS 1668.2 separates fire/smoke, natural ventilation and microbial-control design from its own scope.
Need the Standard applied to real equipment?
AS Applied helps frame the question. Detailed assessment, design, FEA, repair design, verification and RPEQ services are provided through XPO Engineers.